The entire contents of Japanese Patent Application No. 9-298748 filed on Oct. 30, 1997 and Japanese Patent Application No. 10-88787 filed on Apr. 1, 1998 are incorporated herein by reference.
The present invention relates to a computer system having a function of displaying closed caption data included in a data stream that has been digitally compressed and encoded on a display monitor, and a closed caption display method used in that system.
In recent years, along with the development of computers and multimedia techniques, various kinds of so-called multimedia compatible personal computers have been developed. A personal computer of this type has a function of reproducing motion pictures and audio data in addition to text and graphics data.
With the advent of such multimedia computers, a DVD has gained a lot of attention as an alternative storage medium to a CD-ROM. A single DVD-ROM medium can record data of about 4.7 Gbytes about seven times the capacity of the existing CD-ROM on one side, and can record data of about 9.4 Gbytes in case of two-sided recording. Using such DVD-ROM media, titles such as movies including a large volume of video information can be reproduced with high quality on the computer.
Video information recorded on a DVD-ROM medium consists of two kinds of data, i.e., presentation data and navigation data. The presentation data is a set of video objects to be reproduced, and consists of video, subpicture, and audio data. The video data is compressed and encoded by MPEG2. As the coding schemes of subpicture and audio data, runlength coding, AC-3, and the like are supported. The subpicture data is bitmap data, and is used for displaying superimposed dialogues of a movie, selection items on a menu window, and the like. One video object can contain video data for one channel, audio data for a maximum of eight channels, and subpicture data for a maximum of 32 channels.
The navigation data is reproduction control data for controlling the reproduction sequence of the presentation data, and navigation commands can be embedded in this data. The navigation commands are used for changing the reproduction contents or order of video data. Using the navigation commands, a title creator can define various branch structures in his or her title, and can create an interactive title.
When a title stored in a DVD is to be reproduced on a computer, data read out from a DVD-ROM drive is loaded onto a main memory of the computer, and is then transferred to an MPEG2 decoder. The MPEG2 decoder descrambles an encoded video data stream, which has been scrambled to prevent the title from being illicitly copied, and then decodes to expand the encoded video data stream. The decoded video data is displayed on a display monitor or an external TV under the control of a display controller.
A title stored in a DVD is normally created for the purpose of reproduction on a home TV using a home-use player. For this reason, the contents of the title often include closed caption data which is popularly used in a TV video signal, in addition to subpicture data.
The closed caption data is character data superposed on line 21 during the vertical blanking period of the TV video signal, and in the United States, most of home TVs have a function of displaying the closed caption data. Upon displaying the closed caption data, the viewer can confirm the contents of the TV video not only by audio information but also character information. In an MPEG2 stream that forms a DVD title, closed caption character data is called "line 21 data".
As described above, as a method of reproducing a DVD title using a computer, there are two methods, i.e., a method of connecting a home TV to the NTSC output of the computer, and watching the title on that TV, and a method of directly watching the title on a display monitor of the computer.
A case will be examined below wherein a DVD title that contains closed caption character data as line 21 data is reproduced.
When the contents of the DVD title are watched using a home TV, the closed caption character data can be displayed on the TV screen as character information that helps viewing the video, as long as the TV is closed caption compatible. However, when the contents of the DVD title are directly watched on the display monitor of the computer, its display screen is controlled by a display controller which is incompatible with closed caption, and the viewer cannot watch closed caption information.